1. Field of the Invention
The present invention relates to a zoom lens and an optical apparatus using the same and, more particularly, to a zoom lens utilizing the rear-focus method and having a high range with a relatively small number of constituent lenses, suited to be used in video cameras, film cameras and broadcasting cameras, and an optical apparatus using the same.
2. Description of Related Art
For a zoom lens to be used in a photographic camera, a video camera or like optical apparatus, the use of a lens unit other than the front or first lens unit in focusing, or the so-called "rear-focus" method, has been previously proposed in many examples. This is because the rear-focus method admits a lens unit of relatively small size and light weight to move during focusing. Therefore, the driving torque can be weak and, nonetheless, fast focus adjustment can be effected, so that the rear-focus method has an advantage for good adaptation to the auto-focus system.
Such a rear-focus type of zoom lens is disclosed in, for example, Japanese laid-Open Patent Application No. Sho 63-44614, which comprises, in order from an object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power axially movable for varying the focal length, a third lens unit of negative refractive power axially movable for compensating for the shift of the image plane with a variation of the focal length and a fourth lens unit of positive refractive power, totaling four lens units. In this so-called "4-unit" zoom lens, the third lens unit is made movable to effect focusing. However, such an arrangement must assure creation of a large space in which to move the third lens unit. So, the total length of the entire lens system tends to increase greatly.
Japanese Laid-Open Patent Application No. Sho 63-278013, too, discloses a 4-unit zoom lens comprising a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of negative refractive power and a fourth lens unit of positive refractive power, wherein zooming is performed by the second lens unit and compensation for the image shift and focusing are performed by the fourth lens unit. In the zoom lens configuration that has made the third lens unit negative in refractive power, however, because the diverging rays of light from the second lens unit are further diverged by the third lens unit, the diameter of the fourth lens unit becomes larger, causing the of increasing the bulk and size of the entire lens system. Moreover, the range of variation of aberrations due to focusing tends to ever more widen.
On the other hand, Japanese Laid-Open Patent Applications No. Sho 62-24213 (corresponding to U.S. Pat. No. 4,859,042) and No. Sho 63-247316 propose a zoom lens comprising, in order from an object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, totaling four lens units, wherein the second lens unit moves to effect zooming and the fourth lens unit moves to compensate for the image shift with zooming and to effect focusing. Such an arrangement assures minimization of the bulk and size of the entire lens system.
Japanese Laid-Open Patent Application No. Sho 63-29718 discloses a zoom lens comprising, in order from an object side, a first lens unit of positive refractive power, a second lens unit which is constructed with a negative lens, a negative lens and a positive lens, totaling three lenses, whose overall refractive power is negative and which, during zooming, is movable as mainly governing the variation of the focal length, a third lens unit having a positive refractive power and containing an aspheric surface and, after a bit large air separation, a fourth lens unit having a positive refractive power and movable for compensating for the image shift with zooming and for focusing.
Japanese Laid-Open Patent Application No. Hei 5-72472 discloses a zoom lens, using aspheric surfaces, which comprises, in order from an object side, a first lens unit having a positive refractive power and stationary during zooming and focusing, a second lens unit having a negative refractive power and movable for zooming, a third lens unit of positive refractive power fixed and having a light condensing action and a fourth lens unit of positive refractive power axially movable for keeping the constant position of the image plane against zooming, wherein the second lens unit is constructed with a negative lens of meniscus form, a negative lens of bi-concave form and a positive lens, the third lens unit is constructed with a single lens having one or more aspheric surfaces, and the fourth lens unit is constructed with lenses having one or more aspheric surfaces.
In the above-mentioned references, however, there is not disclosed any zoom lens in which the second lens unit is constructed with four lenses. Further, there is not disclosed any arrangement in which an aspheric surface is contained in the second lens unit.
Meanwhile, U.S. Pat. No. 4,299,454 discloses a zoom lens comprising, in order from an object side, a positive first lens unit, a negative second lens unit and a positive rear lens unit, wherein zooming is performed by moving at least two lens units including the negative second lens unit. The negative second lens unit is constructed with, in order from the object side, first and second negative lenses and a positive doublet. However, because the third lens unit is movable, the mechanism therefor results in an increased complexity of structure. U.S. Reissue Pat. No. 32,923 discloses a zoom lens comprising, in order from an object side, a positive first lens unit, a negative second lens unit, a stop, a positive third lens unit and a positive fourth lens unit. The first and fourth lens units are arranged during zooming to move in the same direction, and the stop remains stationary during zooming. Further, the second lens unit contains one cemented lens.
In the two U.S. Patents mentioned above, however, there are no examples suggesting that the third lens unit is constructed with inclusion of a double-aspherical lens and also that any aspheric surface is used in the second lens unit.
Japanese Laid-Open Patent Applications No. Hei 7-270684 and No. Hei 7-318804 disclose a zoom lens comprising, in order from an object side, a first lens unit having a positive refractive power and being fixed, a second lens unit having a negative refractive power and axially movable for varying the focal length, a third lens unit having a positive refractive power and being fixed and a fourth lens unit of positive refractive power axially movable for keeping the constant position of the image plane against zooming and for focusing, wherein the second lens unit is constructed with four single lenses. However, there are disclosed no zoom lenses having a double-aspherical lens in the third lens unit.
Japanese Laid-Open Patent Applications No. Hei 5-060974 discloses a zoom lens comprising, in order from an object side, a first lens unit having a positive refractive power and being fixed, a second lens unit having a negative refractive power and axially movable for varying the focal length, a third lens unit having a positive refractive power and being fixed and a fourth lens unit of positive refractive power axially movable for keeping the constant position of the image plane against zooming and for focusing, wherein the total length of the entire lens system is shortened. However, there are disclosed no zoom lenses both with the use of four single lenses in the second lens unit and with a double-aspherical lens in the third lens unit.